Random fiber laser is a new type of fiber lasers. Compared with conventional fiber lasers, benefiting from its modeless, better stability, higher realibility, simpler structure and many other advantages, random fiber lasers have developed quite well in recent years in realization of a number of novel light sources including high power/high efficiency, broadband, low coherence, etc, leading to many research achievements. The author of this review is the leader of the first group to carry out research on Random fiber lasers in China. The development history of random fiber lasers is firstly summarized systematically. Then, the recent advances in random fiber lasers and their applications to optical fiber sensing and communication are introduced mainly. Finally, the future developments of random fiber lasers are prospected.

Crystal fiber is a new type of high performance fiber material with high concentration of rare earth ions, good light transmission, high temperature resistance and corrosion resistance. It has been recognized that crystal fiber has great potential in laser and sensing applications. However, small core diameter crystal fibers with crystalline core and crystalline cladding have not been successfully fabricated. Compared with traditional glass fiber, the preparation process of crystal fiber is more complicated. How to improve and innovate the crystal fiber preparation process is an important problem that needs to be solved. In order to explore ways to improve the quality of crystal fiber, this paper takes the four preparation techniques of crystal fiber as the main line, reviews the development history of crystal fiber and its preparation method, discusses the limitations of each preparation method. The current application status of crystal fiber is summarized and its development trend is prospected.

Single-crystal sapphire fibers have attracted much attention in high temperature and high pressure environments due to their high melting point, good mechanical properties and stable chemical properties. The preparation method, basic characteristics and high-temperature transmission characteristics of the single-crystal sapphire fiber are introduced. Then, the Fabry-Perot interferometer, Bragg grating and Michelson interferometer based on single-crystal sapphire fiber are discussed. The working principle and implementation methods of these sensors are analyzed. At last, in view of the difficulty of compatibility between single-crystal sapphire fiber and traditional single-mode fiber, the preparation of sapphire-derived fiber and its recent development are introduced. The future development direction of sapphire fiber is also discussed.

Chalcogenide microstructured optical fibers combine the advantages of infrared chalcogenide glass and microstructured optical fibers, which have great applications in the fields of molecular structure identification in infrared fingerprint region, trace gas detection, deep space detection, infrared laser transmission and so on. In this paper, the research progress of infrared chalcogenide microstructured optical fibers is introduced in three aspects: structure classification, application characteristics and fabrication technology. The future development of chalcogenide microstructured optical fibers is also prospected.

Plastic optical fibers have the characteristics of lightweight, softness, low cost and so on. It is noteworthy that plastic optical fibers and other types of fibers have complementary advantages in function, then various plastic optical fibers can be applied in communication, image transmission, illumination, and decorative respectively to realize different functions. In the field of communication, graded index plastic optical fibers have obtained the transmission rate of 10 Gbps@100m in the consumption level, and 40 Gbps in the test level. In the field of image transmission, there are image bundles with 0.45 mm diameter@7 400 pixels and 1.5 mm diameter@13 000 pixels, as well as optical fiber plate products with resolutions up to 256 lp/mm. In the field of plastic optical fiber lasers, the theoretical and experimental research on the relationship between the parameters (gain medium, fiber length, fiber structure) and the characteristics of laser/amplifier is gradually deepening. In the field of decoration and illumination, lots of devices have been developed such as solar optical plastic optical fiber illumination, modeling plastic optical fiber illumination and plastic optical fiber blanket therapeutic instrument. In this paper, the latest researches and applications of plastic optical fiber mentioned above are reviewed.

Recent research progress of rare-earth or transition metal ions doped luminescent chalcogenide (ChG) glass ceramics (GCs) was reviewed. Correlation between emission properties and microscopic distribution of rare earth ions in ChG GCs, and the underlying mechanisms of the interaction between different rare earth ions and crystal field were discussed comprehensively. Moreover, the key features of novel transition metal ions activated ChG GCs developed in the past two years were also summarized. Strong ultrabroadband of Cr2+: 1.8~2.8 μm, and Co2+: 2.5~4.5 μm mid infrared emissions were realized with careful compositional design and controlled crystallization. Our short review summarized the key issues that can influence and restrict the functionalization of ChG GCs, offering a guideline for future research direction on this topic.

The whispering gallery modes microcavity devices play an important role in modern optics. They have wildly applied prospect in many field such as high sensitivity sensor and low threshold laser. However, they are very susceptible to outside interference such as vibration, temperature and humidity, which is challenging to make them practical. In recent years, the whispering gallery modes microcavity devices for practical uses has caused widely concern and many related studies have been reported. In this paper, the latest research progress on packaging and integration of whispering gallery modes microcavity devices is briefly introduced.

With the development of new micromachining processes, the traditional tapered fiber has derived thousands of different microstructured fibers. Among them, S-type fiber taper has been rapidly developed due to its small size, light weight, anti-electromagnetic interference and flexibility to be directly embedded in the structured system. With the intensive study on S-type fiber tapers, fiber optic sensors based on various structures and functions of S-taper have been continuously proposed for measuring temperature, strain, refractive index, humidity and magnetic field, etc., which enriches the types of fiber optic sensors and has deepened people's understanding of the sensing mechanism. This paper comprehensively reviews the research progress of S-type fiber tapers, including sensing principle, preparation method, various sensing structure combinations and sensing applications, and prospects for its development.

Polarization maintaining fiber can reduce the influence of uncontrollable birefringence or polarization mode dispersion caused by environmental disturbance on transmission through artificially introduced high birefringence, and has applications in precision interference sensing, laser systems, optical communication, etc. Compared with traditional solid core fiber which is limited by the intrinsic defect of material, hollow core fiber can confine the optical field in low refractive index air core by specific microstructure design, with low delay, low dispersion, low nonlinearity, high photoinduced damage threshold, anti-interference and high flexibility of filling liquid or gas. Hollow core fiber which have polarization-maintaining feature can not only exert its performance advantages in the above applications, but also show broad prospects in high-power pulsed laser transmission and biochemical analysis application. In this paper, the development of polarization-maintaining hollow core fiber is briefly reviewed, and its design concepts and fabrication techniques are discussed.

As a typical material for radiation detection, glass scintillator plays an important role in various applications, including medical imaging, high-energy physics, environmental monitoring and security inspection. According to structure features and applications, glass scintillator is mainly classified into high-density glass scintillator, low-density glass scintillator and glass-ceramics scintillator. This article describes various types of glass scintillators, focusing on their fabrication procedures, microstructures, optical properties and applications. The outlook of the future developments of the glass scintillators is also discussed.

The ytterbium-doped silica fiber is one of the important basic element of fiber laser and amplifier system. The key to promote the further increase of fiber system power is the performance improvement of ytterbium-doped fibers. In this paper, the power increase and main challenge to power scaling of high power ytterbium-doped fiber laser system are reviewed, and the improvement scheme for the bottleneck problems of laser power climbing is briefly described. The research progresses in improving power restriction based on fiber fabrication technology, optical fiber material and fiber structural design are described in detail. Furthermore, the future research and development tendency of ytterbium-doped silica fiber are prospected.

The surface morphology and microstructure of fiber preform taper zones and fibers with different fluorine content and waveguide structure were characterized by stereomicroscopy, scanning electron microscopy and Raman spectroscopy. The loss and laser transmission efficiency of large-core fibers with fluorine-doped cladding were analyzed by fiber analysis system and self-made output laser probe. The results show that the fluorine volatilization phenomenon becomes more obvious with the increase of fluorine content. As a result, the surface defects of traditional large-core fibers with fluorine-doped cladding increase, such as cracks and pits. Simultaneously, the fiber loss increases slightly and the laser transmission efficiency decreases. Moreover, the fluorine volatilization and crystallization during the fabrication process of large-core fibers with fluorine-doped cladding are effectively suppressed through subsidence fluorine-doped inner cladding design. Thus, the fiber loss at 1 200 nm is 3.99 dB/km, and the laser transmission efficiency at 2 μm of flat and spherical fibers is 88.9% and 88.4%, respectively. It can be obviously seen that its property is better than that of traditional fibers.

A train positioning and speed measuring method based on grating array was proposed to solve the monitoring problems of railway and urban rail transit, such as variable operating environment, long monitoring distance and vulnerable to lightning. The ultra-weak grating array with strong anti-electromagnetic interference ability and large multiplexing capacity is used to form vibration sensing network, and the unbalanced Mach-Zehnder interference demodulation method is used to realize the detection of train traveling vibration signal. The accurate positioning of the train is realized by the endpoint detection method based on the short-term energy after the wavelet denoising, and the running speed of the train in the area is calculated by the distance between the adjacent measuring areas and the arrival time difference of the train. An experimental system was set up between Hugong and Yezhihu stations of Wuhan Metro Line 7 for field test to verify the feasibility of this scheme. The experimental results show that this scheme can achieve accurate positioning of the real-time position of the train, with the measuring speed error is ±2 km/h. And after a long time of testing, the system based on this scheme has good reliability and stability.

A high repetition rate all-fiber picosecond laser which mode-lacked by a semiconductor saturable absorber mirror is demonstrated. Stable mode-locked pulse train is obtained when the pump power is 112 mW in the ring cavity. The pulse width of the output laser is 4.2 ps at the repetition rate of 19.2 MHz, and it exhibits a spectral bandwidth of 3.6 nm at the center wavelength of 1 064.1 nm. Due to the length of the cavity, it is difficult to increase the repetition rate of the fiber laser. So a novel low-loss high-repetition-rate pulsed laser modulator based on optical coupler and time delay fiber is introduced to increase the repetition rate of the seed to 1.2 GHz. This design effectively reduces the energy loss of the pulse during coupling, which provides a new way to boost the repetition rate of the all-fiber ultra-short pulsed laser.

A Forked Photodiode Array (F-PDA) electrodes were presented to address the issues brought by transmission delay of traditional Traveling-Wave Phohodiode Array(TW-PDA), which cause phase-mismatches of electrical signals and affect the overall frequency response. This design overcomes the problems cited above by using a symmetric-connected structure instead of the cascade-connected structure of TW-PDA to ensure that the terminal pad is equidistant from each unit. The structural parameter of F-PDA is simulated and optimized by using simulation software HFSS. In the simulation, the characteristic impedance is kept at about 50 Ω and the insertion loss is less than 1.5 dB within the boundary of 100 GHz. Traditional and forked structures were fabricated and tested in experiment. The results show that the forked structure has a better performance compared with the traditional structure. At 40 GHz, the later has an insertion loss of 2.5 dB and a 3 dB bandwidth of 6.9 GHz, correspondingly, the properties of the former are 1.36 dB and 13.8 GHz.

A photonic integrated radio frequency self-interference cancellation chip is designed for full-duplex communication. The radio frequency signals are converted into the optical domain by phase modulation, and the amplitude and phase tuning of the optical carried radio frequency signals is implemented to realize the radio frequency self-interference cancellation function. The optimally designed structural parameters of the cascaded ring are presented. The time delay tuning range is 0 to 10 ps with the delay jitter less than 0.1 ps over 30 GHz bandwidth. For filtering response, the stopband rejection is 36.5 dB, the passband bandwidth is 60.6 GHz, and the steepness of the edge is 9.2 dB/GHz. The theoretical model of the photonic integrated radio frequency self-interference cancellation system is established. The influence of time delay and amplitude mismatch caused by optical delay line, tunable optical attenuator and filter on the radio frequency self-interference cancellation performance is analyzed. With the amplitude mismatch of 0.02 dB, the cancellation depth is -42.7 dB over the bandwidth of 2 GHz. With the time delay deviation of 0.07 ps, the cancellation depth is -37 dB over the bandwidth of 2 GHz. This work paves the way for fabricating the photonic integrated function chip.

A novel fiber used in extreme low temperature (-70℃) was designed, which includes the core layer, the inner cladding layer, the depression cladding and the outer cladding layer. The relationship between the bending loss and the refractive index difference was studied. By comparing the thickness and modulus of different coating layers, optimizing design of the fiber drawing process, a quartz fiber that can be used in -70℃ for a long time was manufactured. The experimental results show that the additional loss (-70℃) of the fiber at 1 550 nm and 1 625 nm is less than 0.01 dB/km, which provides reliable theoretical and experimental basis for the preparation and industrialization of extreme cold fiber and optical fiber composite overhead ground wire.

The electro-optic switch array device with N+1 horizontal channels, N vertical channels and 2N microrings is designed, and the transmission characteristics are analyzed. When the operation voltage is applied on the microrings in the array unit in various ways, the switching function of the corresponding horizontal channel can be realized. Taking the structure of 5×4 channels and 8 microrings as an example, the width and thickness of the waveguide core are chosen as 1.5 μm, the thickness of the buffer layer between the waveguide core and electrode is set as 2.0 μm, the thickness of the metal electrode and the coupling gap between the microring and channel are selected as 0.05 μm and 0.1 μm, respectively. When the switching voltage is 8 V, the insertion loss of the structure ranges between 2.79 and 4.31 dB, the crosstalk is below -20 dB，and the extinction ratio is over 12.5 dB.

(Ge10Te43)90-AgI10 and Ge10Sb10Se80 glasses were prepared using melt quenching method, and an environmentally friendly suspended-core fiber based on the above two glasses was fabricated via a extrusion technique. The fiber exhibits excellent transmission window ranging from 3 to 11 μm with a minimum loss of 0.5 dB/m, the optical energy and beam spot of the suspended-core fiber were also investigated. In addition, a supercontinuum spanning from 1.6 μm to 8.9 μm was obtained as pumped by a mid-infrared optical parametric amplifier.

A sausage-like whispering-gallery-mode microresonator was experimentally investigated, whose optical transmission spectrum is measured. The experimental results show that when the sausage-like microresonator is stretched from its two ends, the resonance wavelengths of the optical modes will decrease, and the speeds of the decrease of the resonance wavelengths of different modes are different. By stretching the sausage-like microresonator, the experimental observation of Fano resonance phenomenon has been achieved. In addition, the related electromagnetically induced transparency-like phenomenon has also been observed in experiment. The quality factor of the whispering-gallery mode involved in the experiment can be as high as 108.

In order to realize the high sensitivity measurement of the liquid refractive index, a sensitivity-enhanced all-fiber-optic liquid refractive index sensor based on vernier effect is proposed and demonstrated. The sensor is composed of a Fabry-Perot sensing cavity and a Fabry-Perot reference cavity in parallel. The sensing cavity is an open cavity made by offset splicing of single mode fiber, and the reference cavity is a sealed cavity, which is fabricated by fusion splicing of single mode fiber and hollow optical fiber. The free spectral ranges of the two Fabry-Perot cavities are similar, but not equal. Vernier effect is generated when the beams are superimposed via a fiber coupler to achieve the sensitization effect. Experimental results show that the refractive index sensitivity of the proposed sensor is about 9 048.78 nm/RIU, which is about 8 times that of a single sensing cavity. Due to the advantages of high sensitivity, simple structure, easy preparation and low cost, the sensor has potential application foreground in chemical and biomedical domain.